Method of producing finely divided calcium carbonate which does not agglomerate



Patented Dec. 13, 1938 NEIE IHOD OF PRODUCING FINELY DIVIDED CALCIUMCARBONATE WHICH DOES NOT AGGLOMERATE Edward M. Allen, Barberton, andGeorge M. Lynn, deceased, late of Wadsworth, Ohio, by Ethel P. Lynn,administratrix, Wadsworth, Ohio, assignors to Pittsburgh Plate GlassCompany, a corporation of Pennsylvania No Drawing. Original applicationJanuary 21,

1933, Serial No. 652,920. Divided and this application January 15, 1936,Serial N 0. 59,312

9 Claims.

The primary object of this invention is to produce calcium carbonate ofvery fine particle size which has special value as a pigment in therubber, putty, and paint industries. By fine calcium carbonate we mean,having a particle size substantially under one micron in distinctionfrom the present by-product calcium carbonates which are mostly aboveone micron in average particle diameter.

There are three commercially possible methods of making precipitatedcalcium carbonate: By mixing together solutions of a soluble calciumsalt and a soluble carbonate; by mixing together carbon dioxide gas andcalcium hydroxide; and by mixing calcium oxide or hydroxide and asoluble carbonate. Of these, the third is the most advantageouseconomically in that it produces a valuable by-product, the hydroxidecorresponding to the soluble carbonate.

In the usual process for producing caustic soda by reaction of sodiumcarbonate with calcium hydroxide, the carbonate, dissolved in water, andthe lime, in the form of a slurry, are brought together. Alternatively,either quicklime or solid slaked lime is added to a sodium carbonatesolution. In order to effect rapid causticization and good conversion,and to obtain a coarse carbonate with a good rate of settle, thereaction is always practiced at a high temperature. The calciumcarbonate is then settled out, washed free of caustic, and eitherdiscarded or reburned. A small proportion is sold as by-product whiting.Our invention so alters the value of the calcium carbonate, that itbecomes the primary product with caustic alkali as the by-product, whichis just the reverse of the usual case. However, the practice of ourinvention produces a caustic in no way inferior to that produced in theusual procedure.

A certain amount of the usual by-product whiting is used in the rubberindustry as an inert filler. In a well balanced rubber stock, such as iscommonly used for automobile tire inner tubes, this by-product whitinggives a tensile strength at best cure of approximately 2400 pounds persquare inch. Such by-product whiting is a relatively coarse pigment foruse with rubber, since the particles range from 0.2 micron to 30 micronsin diameter, with the greater weight per cent in the range 2.5 to 10microns.

By our improved process a product is produced of which usually about 95weight per cent has a particle size of less than 0.5 micron in diameter.When used in the same rubber stock, our product gives a tensile strengthof approxi- In accordance with our invention, an aqueous slurry offinely divided calcium carbonate is prepared by suitable methods and theslurry is then treated with suitable coating agents in order to preventcementation during drying as hereinafter more fully described. Thus, inan illustrative modification of our process, we slake the lime carefullyin hot, preferably boiling, -water, adding the lime to the water at sucha rate as to keep the slurry boiling. Vigorous agitation is desirable atthis stage. The resultant lime possesses a very high specific surface(high surface per unit weight) most of the particles ordinarily beingsmaller than one micron. It will be understood that the specific surfaceis the ratio between the absolute surface of solute to its total volume.The finer the subdivision, the greater the specific surface and thehigher the dispersion of the material in solution. Substantially all ofthe particles are less than one micron in some one dimension. Between0.5 and 1.5 gallons of water may be used to each pound of quicklime. Thequicklime should preferably be in small pieces so that rapid slaking maybe effected. We believe that the combination of high temperature withthe consequent rapid slaking, and vigorous agitation prevents the growthof large calcium hydroxide crystals, such as are ordinarily formed bycold slaking of lime.

While the procedure described above is the preferred method of slakingto give fine lime, we have found that fine lime capable of being reactedwith alkali carbonate to give fine calcium carbonate can be made byslaking lime in cold Water with vigorous agitation and afterwardsbeating the milk of lime vigorously and for a vigorous agitation, thegreater is the amount of work which must be expended in beating to bringthe lime to a proper state for making fine calcium carbonate.

The lime slurry is then diluted to approximately 0.5 pound of CaO pergallon, and cooled substantially to room temperature. If the quicklimeused is not of high quality, there may be a small amount of relativelycoarse material in the lime slurry. If such coarse material is present,it is desirable to remove it at this stage in any convenient manner. Wehave found screening through a 300 mesh screen to be satisfactory, butdo not limit ourselves to this.

The lime slurry is then treated with an excess of sodium carbonate.Enough soda ash usually is added to give from 25 to 50 per cent excessover the stoichiometric ratio, although it is not essential to observethese limits. The soda ash is added rapidly and vigorous agitation isessential at this time. A rise in temperature ranging from 10 degrees C.to 15 degrees C. occurs immediately following the addition of the sodaash.

There appears to be a balance between the work put into the system andthe fineness of the calcium carbonate produced. If the milk of lime issufficiently fine and the agitation adequate, very fine calciumcarbonate will be produced, temperature and concentration beingsuitable. Particles of larger size can be produced if a suitablydecreased degree of agitation is employed. While some fine particleswill be produced, the bulk of the. material can be formed with a uniformparticle size, the diameter of the particles being controlled by thedegree of agitation. On the other hand, by increasing the degree ofagitation over that used for milk of lime prepared under the preferredconditions, fine calcium carbonate can be produced from milk of limesomewhat less fine than the preferred state. The amount of worknecessary to convert lime less fine than the preferred state increasesrapidly with decrease in fineness of the lime and a limit is placed onthe coarseness of the lime by the practical difficulties involved inincreasing the degree of agitation unduly.

We believe that the combination of lime of high specific surface andsoda ash with a. fast rate of solubility creates a high concentration ofcalcium ions and of carbonate ions, which under the conditions ofvigorous agitation react together to produce a very large number ofcrystal nuclei. The relatively low temperature at which thecausticization reaction is eflected aids in attaining a higherconcentration of calcium ions in the solution, since calcium hydroxideis more soluble in cold water than in hot. In consequence of thesefactors, the reaction product is composed of a very large number of verysmall crystals. The reaction mixture is then filtered and the calciumcarbonate washed in any convenient manner. Washing is continued untilthe calcium carbonate is free, or substantially free, of caustic. Thecalcium carbonate may be finaly repulped with water and treated with acoating agent as described below.

We have found that with suitable milk of lime containing 0.5 pound ofcalcium oxide per gallon and at a temperature between 25 degrees C. and30 degrees C., with moderate stirring a gel structure is usually formedin the reaction mixture. The mixture stiffens, then sets to a fairlystiff gel in from two to ten minutes, the exact time of gelling beingdependent on the concentration of the milk of lime, its fineness, theconcen- -tration of the sodium carbonate after it goes into solution,the temperature, and the degree of agitation. We believe the gelstructure is due to the formation of an extremely large number of fine,usually 98 per cent being smaller than one micron in diameter. Atextremely high magnification they appear to be mostly stubby prisms ofvery uniform size. We have found that the breaking up of the gel-formthus at high pH, inhibits the crystal growth of calcium carbonate. Whenthe small crystals are once formed under these conditions, they havelittle tendency to grow in water suspension.

We have further discovered that the use oi more vigorous agitation willprevent the formation of the actual gel'structure in the reactionmixture. The mechanical force applied in the agitation 'ap-parentlykeeps the incipient gel structure broken up. The reaction mixtureincreases considerably in viscosity but can be prevented from actuallyforming a stiff gel by sufiiciently vigorous agitation. The calciumcarbonate formed under such conditions is essentially of the samequality as if the gel were allowed to form under more moderateagitation.

Furthermore, we have observed that raising the temperature, otherconditions being the same, tends to prevent gel formation. However,decreasing the agitation may oifset this temperature effect sufficientlyto allow the gel to form. Thus a mixture which would gel at 25 degreesC. to 28 degrees (1., was found not to do so at 40* degrees C. However,when the agitation was sufficiently decreased, it did gel at 40 degreesC.

We have discovered that under the conditions of concentration andtemperature under which a gel structure will form with no agitation oronly very slight agitation, very fine particle-size calcium carbonatecan be formed if vigorous agitation is used.

It has also been determined that if the proper conditions of temperatureand concentration are observed, very fine particle-size calciumcarbonate can be prepared from milk of lime and a solution of sodiumcarbonate. For example, working at a temperature between 25 degrees C.and 40 degrees C., and at a concentration of 0.5 to 1.5 pounds of ofcalcium carbonate per gallon of the reaction mixture, we have produced asatisfactory product in this Way. In this case, of course, the milk oflime must be more concentrated in order that the water necessary todissolve the sodium carbonate may not dilute the final reaction mixturebeyond the limits at which a gel can form under conditions of noagitation or only very slight agitation.

We have found that conditions which will insure the formation of veryfine particle-size calcium carbonate are: that suitable lime isnecessary, as heretofore described; that the reaction between the milkof lime and sodium carbonate should take place within the temperaturelimits 25 degrees C. to 40 degrees (2.; that the final reaction mixtureshould have a concentration of between 0.5 to 1.5 pounds of calciumcarbonate per gallon; and that vigorous agitation should be applied tothe reaction mixture.

It has been determined that treatment with a coating agent besidespreventing cementation on drying also gives a more friable product, andimproves the milling properties of a rubber compound embodying thematerial as a pigment. Such a treated pigment has a tendency to ride thebank on the rubber mill, thereby diminishing the amount of laborrequired to incorporate the pigment into the rubber batch. Many rubberpigments go through the bite of the mill into the pan and have to beshoveled back onto the rolls, only a small portion of the pigment beingincorporated into the rubber with each pass of the bulk of the pigmentthrough the mill. The property which this improved pigment possesses ofriding the bank materially decreases the shoveling required and sodecreases the labor of The treatment of the material with a coatingagent also causes the particles to cling together to a sufiicient degreeto decrease the pigment loss by flying, many fine rubber pigments havinga tendency to float in the air and thereby cause waste. The tendency ofthe particles to cling together is sufficient to decrease this waste toa marked degree, but not so strong as to interfere with ready andcomplete separation of the particles by the rubber such as is desirablefor good dispersion.

The following materials have been discovered to be efiicient as coatingagents: The higher saturated fatty acid esters of mono, di, and tribasicalcohols such as butyl, glycol and glyceryl stearates and naturallyoccurring mixtures of esters as found in cocoanut oil, lard, lard oil,corn oil, sperm oil, tallow, etc. These oils are commonly known asnon-drying oils. These agents are compatible with rubber or plastics, orwith paint, lacquer, and putty vehicles. Vigorous agitation or beatingis used to insure uniform dispersion of the coating agent on the surfaceof the particles. The slurry is then filtered, and the cake dried anddisintegrated.

The following examples are typical of a practical application of ourinvention:

I. Three pounds of quicklime (burned marble, burned limestone, or othersuitable burned calcium carbonate), are slaked by being added withadequate stirring to two gallons of hot water at such rate as tomaintain the mixture near the boiling point. With the third pound oflime, about a gollon of hot water is gradually added so as to keep themixture sufficiently fluid for good agitation. Stirring is continued forsome time, usually about fifteen minutes, after all the lime has beenadded. The slurry is then diluted to approximately five gallons, cooledto about room temperature, screened or classified to remove any coarsematerial, and treated with seven and onehalf pounds of light soda ash,the vigorous stirring, which is continued for about thirty minutes.During that time the reaction mixture becomes very stiff, then forms agel and finally again becomes fluid. It is filtered and washed to removethe soluble salts. When the cake is substantially free from caustic itis repulped in water and one hundred and ten grams of coconut oil isadded with continued vigorous stirring to distribute the oil uniformlyover the surface of the particles.

II. Three pounds of lime is slaked as in Example I, diluted, freed fromcoarse material, cooled to approximately 35 degrees C., and treated withseven and one-half pounds of light soda ash, with vigorous stirring,which is continued for about thirty minutes. During that time thereaction mixture passes through a period of markedly higher viscositywithout actual gel formation, and again becomes more fluid. It isfiltered and washed to remove the soluble salts. When the cake issubstantially free from caustic it is repulped in water and one hundredand ten grams of coconut oil is added with continued vigorous stirringto distribute the oil uniformly over the surface of the particles.

III. Three pounds of lime are slaked as in Example I, dilutedsufiiciently to enable any coarse material to be removed, thickened toabout 2 gallons of slurry and mixed with a solution of seven andone-half pounds of sodium carbonate in about three gallons of Water, ata temperature of about 30 degrees C., and with vigorous agitation whichis continued for about thirty minutes the reaction mixture in thisinterval increases and then again decreases in viscosity. The slurry isfiltered and washed. When the cake is substantially free from caustic itis repulped in water and one hundred and ten grams of coconut oil isadded with continued vigorous stirring to distribute the oil'uniformlyover the surface of the particles.

The slurry from any of the examples is again dewatered, the cake driedand crumbled. If the coating agent is omitted, the particles may cementthemselves together in the cake and vigorous treatment will be necessaryto break the aggregates down into the primary particles. This issometimes done in the practice of this invention in the event thatuncoated fine whiting is desired.

It will be noted that there are four variables which may be controlledin successfully producing calcium carbonate of fine particle size, Thevariables are quality of the milk of lime, concentrations, temperatureand degree of agitation.

The limits of variation of any one are fixed by' values of the otherthree. Manifestly it is impossible to define exactly limitations for anyone of the variables, particularly since a statement of the degree ofagitation is a. comparative matter at best. Such state of facts shouldbe borne in mind in applying restrictions to interpretation of the scopeof the invention herein described and claimed.

This case is a division of our application Serial No. 652,920, filedJanuary 21, 1933.

What we claim is:

1. A method of producing calcium carbonate of fine particle size andadapted for use as a pigment in rubber, which consists in reacting milkof lime of high specific surface with sodium carbonate at an initialtemperature between 25 C. and 40 C., vigorously agitating the resultingmixture to produce simultaneously sodium hydroxide and finely dividedcalcium carbonate, 98 per cent of the particles of said calciumcarbonate being of a size less than one micron washing out the sodiumhydroxide to obtain a slurry of calcium carbonate substantially free ofsodiumhydroxide, adding coconut oil to the slurry, vigorously agitatingthe mixture in order to coat uniformly the particles of calciumcarbonate with the coconut oil, filtering the slurry and drying theresultant product.

2. A method of producing calcium carbonate of fine particle size andadapted for use as a pigment in rubber, which consists in reacting milkof lime of high specific surface with sodium carbonate at an initialtemperature between '25 C. and 40 C., vigorously agitating the resultingmixture to produce simultaneously sodium hydroxide and finely dividedcalcium carbonate, 98 per cent of the particles of said calciumcarbonate being of a size less than one micron washing out the sodiumhydroxide to obtain a slurry of calcium carbonate substantially free ofsodium hydroxide, adding approximately 5% by weight of coconut oil tothe slurry, vigorously agitating the mixture in order to coat uniformlythe particles of calcium carbonate with the coconut oil, filtering theslurry and drying the resultant product.

3. A method of preparing substantially dry, substantially unagglomeratedfinely divided calcium carbonate 98 per cent of the particles of saidcalcium carbonate being of a size less than one micron which comprisesprecipitating said finely divided carbonate in a body of water to forman aqueous slurry thereof incorporating coconut oil in said slurry, anddrying the resulting mixture, the proportion of said oil beingsufficient to coat a sufiicient portion of said calcium carbonateparticles to prevent substantial agglomeration thereof during the dryingtreatment.

4. A method of preparing substantially dry, substantially unagglomeratedfinely divided calcium carbonate having an average particle size underone micron, which comprises precipitating said finely divided carbonatein a body of water to form an aqueous slurry thereof, incorporatingcoconut oil in said slurry and drying the resulting mixture, theproportion of said oil being sufficient to coat a sufiicient portion ofsaid calcium carbonate particles to prevent substantial agglomerationthereof during the drying treatment.

5. A method of preparing substantially dry, substantially unagglomeratedfinely divided calcium carbonate, 98 percent of the particles of saidcalcium carbonate being of a size less than one micron, which comprisesprecipitating said finely divided carbonate in a body of water to forman aqueous slurry thereof, incorporating an oil of the group consistingof coconut oil, lard, lard oil, corn oil, sperm oil and tallow in saidslurry con taining said carbonate and drying the resultant mixture, theproportion of said oil being sufficient to coat a suflicient portion ofsaid calcium carbonate particles to prevent substantial agglomerationthereof during the drying treatment.

6. A method of producing calcium carbonate of fine particle size andadapted for use as a pigment in rubber, which consists in reacting milkof lime of high specific surface with an alkali metal carbonate at aninitial temperature between 25" C. and 40 C., vigorously agitating theresulting mixture to produce simultaneously an alkali metal hydroxideand finely divided calcium carbonate, 98% of the particles of saidcalcium carbonate being of a size less than one micron, washing out thealkali metal hydroxide to obtain a slurry of calcium carbonatesubstantially free of an alkali metal hydroxide, adding an oil of thegroup consisting of coconut oil, lard, lard oil, corn oil, sperm oil andtallow to the slurry, vigor ously agitating the mixture in order to coatuniformly the particles of calcium carbonate with the coconut oil,filtering the slurry and drying the resultant product.

'7. The process of claim 4 wherein the calcium carbonate is washed priorto addition of coconut oil thereto.

8. A method of preparing substantially dry, substantially unagglomeratedfinely divided calcium carbonate having an average particle size underone micron, which comprises precipitating said finely divided carbonatein a body of water to form an aqueous slurry thereof, incorporating anoil of the group consisting of coconut oil, lard, lard oil, corn oil,sperm oil, and tallow in said slurry and drying the resulting mixture,the proportion of said oil being sufficient to coat a sufficient portionof said calcium carbonate particles to prevent substantial agglomerationthereof during the drying treatment.

9. The process of claim 8 wherein the calcium carbonate is Washed priorto the addition of the oil thereto.

EDWARD M. ALLEN. ETHEL P. LYNN. Administmtrizc of the Estate of GeorgeM. Lynn,

Deceased.

